Copy participant via web service in survey flow

I would like to add a participant from the directory to a specific project as part of a survey flow, but for some reason I can’t make it work.

I have this setup:

It should be straight forward, but I struggle to get the body part right. 

According to the documentation:

https://api.qualtrics.com/c03ae4b3e935d-copy-participants

It should be like this:

[
  "1234"
]

I get an “body should be an array type.” error, but the JSON selection should create this.

Any help would be much appreciated!